During immune responses against infectious agents, the immune system chooses appropriate effector mechanisms from a wide variety of possibilities. Successful responses include effector mechanisms that destroy the pathogen, without causing excessive damage to host tissues. T helper cells expressing different patterns of cytokines play a major role in regulating the choice of effector function. T-cells initially express mainly Interleukin 2 (IL-2), and then differentiate over a few days into cells secreting different sets of cytokines, such as the Thl pattern [Interferon gamma (IFNg) and lymphotoxin (LT)] or the Th2 pattern (IL-4, IL-5, IL-10). These cytokine patterns regulate different types of immune effector function, and the choice between Thl and Th2 responses can be crucial for resisting certain infections. T-cells expressing additional patterns of cytokines are induced during certain infections, but the function of these phenotypes are less understood. Recent data suggest that many T-cells that proliferate during immune responses remain as precursor cells without differentiating into Thl or Th2 cells. These cells may provide an expanded pool of antigen-specific T-cells that can rapidly differentiate into appropriate effector phenotypes during subsequent responses. The two main goals of this project are to determine whether these primed precursor cells provide flexibility for future immune responses, and whether these cells also have effector functions in their own right. The ability of these cells to differentiate into Thl, Th2 and other phenotypes in vitro and in vivo will be determined by tracking normal and TCR transgenic T-cells using a new multicolor Fluorispot assay to analyze the simultaneous secretion of several cytokines from individual cell. To guide the analysis of the functions of the primed precursor cells, gene chips will be used to define the gene encoding proteins secreted these cells, and the ability of the precursor cells to help B-cells and induce inflammation will be tested directly. Knowledge of the differentiation and functions of these additional phenotypes may help us understand immune responses not adequately explained by Thl and Th2 cells. If the precursor cells are flexible with regard to future differentiation, this may be an important population to target for modification during vaccination for chronic diseases in which there is an ongoing immune response, such as allergy, autoimmunity and cancer.